In Long Term Evolution (LTE), the concept of downlink Multiple Input Multiple Output (MIMO) antennas is used to help achieve high peak data rates. In an LTE system, multiple antennas are used at the transmitting eNode B (eNB) and also at the receiving User Equipment (UE). e Node B is the term used in UMTS to denote the Base Transceiver Station (BTS) in an LTE system. By using multiple antennas at both the eNB and UE, the data rates of one individual UE are able to be increased given the appropriate channel conditions.
To further enhance the overall network capacity, further extensions of Multi-user MIMO (MU-MIMO) technology are being further extended in LTE-Advanced systems. As the name suggests, MU-MIMO technology utilises a particular time and frequency resource to serve more than one UE while transmitting with more than a single antenna both at the eNB and UEs. To distinguish between the single and multi-user scenario, the terms SU-MIMO and MU-MIMO are used.
The general idea of SU and MU-MIMO is illustrated in FIGS. 1 and 2 respectively. FIG. 1 depicts an eNB 10 in SU-MIMO communication with UE 12, whereas FIG. 2 depicts an eNB 14 in MU-MIMO communication with UEs 16 and 18.
To facilitate the use of higher order MIMO without incurring serious overhead, the use of dedicated reference signals (DRS) for demodulation (DM-RS) has been adopted by the 3rd Generation Partnership Program (3GPP). Within the SU-MIMO context, DM-RS implies that reference signals contained within the resource block designated to the particular UE are to be used solely by that UE. Although the structure of the DM-RS has yet to be decided, schemes using a combination of frequency division and code division multiplexing are being considered. These are illustrated in FIGS. 3 to 5, which respectively depict resource blocks 20 to 24. The resource blocks 20 to 24 respectively depict DM-RS patterns for up to two, four and eight transmission layers.
Within the MU-MIMO context however, the issue is more complicated. Since the same time and frequency resource is shared by more than one UE, the DM-RS could also potentially be shared by different UEs. This however, would increase the cross interference from one UE to another thereby degrading the channel estimation and overall throughput performance of each UE.
To circumvent this problem, 3GPP has adopted the use of different UEs being mapped to different layers. For example, if the eNB has a total of 4 layers available and needs to serve two UEs using the same time and frequency resources, it can use the first two layers to transmit data to the first UE while the last two layers can be used to serve the second UE. Using such a mapping would therefore imply that each UE would use the DM-RS from the layers it is assigned in order to estimate the channel.